drm/amdgpu: change pp_dpm clk/mclk/pcie input format.

1. support more than 8 values when setting get_pp_dpm_mclk/
sclk/pcie, the former design just parse command format like
"echo xxxx >　pp_dpm_sclk" and current can parse "echo xx xxx
 xxxx > pp_dpm_sclk" whose operation is more user-friendly
and convinent and can offer more values;
2. be compatible with former design like "xx".
3. add DOC: pp_dpm_sclk pp_dpm_mclk pp_dpm_pcie
Bug:KFD-385

Signed-off-by: welu <wei.lu2@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>